1. Field of the Invention
The present invention relates to a method for making a polarization rotator and the polarization rotator made thereby.
2. Description of the Related Art
Japanese Unexamined Patent Application Publication No. 2010-88110 describes a polarization converter (polarization rotator). The polarization rotator includes an asymmetric core constituted by a dielectric waveguide and a cladding in which the asymmetric core is embedded. The asymmetric core of the polarization rotator has a cutaway shape to present an inclined surface over the length thereof (in the waveguide direction). The inclined surface of the asymmetric core forms a predetermined acute angle with a surface on which the polarization rotator is disposed (e.g., a surface of a substrate). Japanese Unexamined Patent Application Publication No. 2010-88110 states that because of the asymmetric shape of the core, the polarization rotator can rotate a plane of polarization guided therein.
A polarization rotator is used, for example, in a polarization division multiplexing optical transmission system to convert a polarization state of a polarized optical signal. For example, an optical signal having a transverse electric (TE) polarization can be converted to an optical signal having a transverse magnetic (TM) polarization by rotating a plane of polarization of the optical signal using the polarization rotator.
In the polarization division multiplexing optical transmission system described above, semiconductor optical devices are generally used for transmitting and receiving optical signals. Specifically, a semiconductor light-emitting device, such as a semiconductor laser, is used as an optical transmitter. On the other hand, a semiconductor light-receiving device, such as a semiconductor photodiode, is used as an optical receiver. For size reduction of the polarization division multiplexing optical transmission system, it is preferable that the polarization rotator and the semiconductor optical devices for transmitting and receiving optical signals be monolithically integrated on the same semiconductor substrate.
However, the polarization rotator described in Japanese Unexamined Patent Application Publication No. 2010-88110 is made of a dielectric material. It is difficult to monolithically integrate, on the same semiconductor substrate, the semiconductor optical devices and the polarization rotator made of a material other than a semiconductor material. Therefore, this polarization rotator is generally inserted in an optical waveguide, such as an optical fiber. If the semiconductor optical devices and the polarization rotator made of materials with different refractive indices are monolithically integrated on the same semiconductor substrate, the optical axes of the semiconductor optical devices and the polarization rotator need to be aligned precisely on the micrometer scale. Moreover, light reflected and returned from an optically coupled portion between the polarization rotator and the semiconductor optical devices may adversely affect the operation of the semiconductor optical devices.
As a solution to this, a polarization rotator having the same shape as that of the polarization rotator described in Japanese Unexamined Patent Application Publication No. 2010-88110 may be formed, using a semiconductor material, integrally with the semiconductor optical devices. This may solve the problems associated with the alignment of optical axes and the reflected and returned light that arise when the polarization rotator and the semiconductor optical devices are monolithically integrated on the same semiconductor substrate.
However, a polarization rotator having the above-described shape and made of a semiconductor material is not known. Generally, semiconductor optical devices are made on a semiconductor substrate by using an epitaxial growth method. Therefore, to integrally form a polarization rotator and the semiconductor optical devices, the epitaxial growth method may be used to form the polarization rotator. However, it is difficult to growth a semiconductor layer in a desired shape on a semiconductor substrate, the semiconductor layer forming an acute angle of inclination with a surface of the semiconductor substrate. This means that it is difficult to form a polarization rotator having the same shape as that of the polarization rotator described in Japanese Unexamined Patent Application Publication No. 2010-88110 using a semiconductor material. Japanese Unexamined Patent Application Publication No. 2010-88110 does not mention any technique that forms a polarization rotator of semiconductor material.